In current systems for direct injection of fuel of a common-rail type, a low-pressure pump supplies the fuel from a tank to a high-pressure pump, which in turn supplies the fuel to a common channel or “common rail”. Connected to the common rail are a series of injectors (one for each cylinder of the engine), which are cyclically driven so as to inject part of the fuel under pressure present in the common rail within the respective cylinders. For proper operation of combustion, it is important that the value of the pressure of the fuel within the common rail should always be kept at a desired value, which may generally vary as a function of the engine point.
In order to keep the value of the pressure of the fuel within the common rail at the desired value, it has been proposed to size the high-pressure pump to supply the common rail with an amount of fuel exceeding the effective consumption in every condition of operation. Coupled to the common rail is an electromechanical pressure regulator, which keeps the value of the pressure of the fuel within the common rail at the desired value by discharging the fuel in excess to a recirculation channel that re-introduces said excess fuel upstream of the low-pressure pump. An injection system of this type presents different drawbacks, in so far as the high-pressure pump must be sized for supplying to the common rail an amount of fuel that is slightly in excess of the maximum possible consumption. However, said condition of maximum possible consumption occurs somewhat rarely and in all the remaining conditions of operation the amount of fuel supplied to the common rail by the high-pressure pump is much greater than the actual consumption, and hence a considerable portion of said fuel must be discharged by the pressure regulator into the recirculation channel. The work performed by the high-pressure pump to pump the fuel that is subsequently discharged by the pressure regulator is “useless” work. Hence, this injection system presents a very low energy efficiency. Furthermore, this injection system tends to overheat the fuel, in so far as, when the fuel in excess is discharged by the pressure regulator into the recirculation channel, the fuel itself passes from a very high pressure to a substantially ambient pressure and, as a result of said pressure jump, heats up.
In order to solve the problems described above, it has been proposed to use a high-pressure pump with variable capacity capable of supplying the common rail only with the amount of fuel necessary for keeping the pressure of the fuel within the common rail at the desired value.
For example, the patent application No. EP0481964A1 describes a high-pressure pump provided with an electromagnetic actuator, which is able to vary instant by instant the capacity of the high-pressure pump by varying the instant of closing of an intake valve of the high-pressure pump itself In other words, the capacity of the high-pressure pump is varied by varying the instant of closing of the intake valve of the high-pressure pump itself. In particular, the capacity is decreased by delaying the instant of closing of the intake valve and is increased by anticipating the instant of closing of the intake valve.
A further example of a high-pressure pump with variable capacity is provided by the U.S. Pat. No. 6,116,870A1. The high-pressure pump described in U.S. Pat. No. 6,116,870A1 comprises a cylinder provided with a piston having a reciprocating motion within the cylinder, an intake channel, a delivery channel connected to the common rail, an intake valve designed to enable passage of a flow of fuel entering the cylinder, a unidirectional delivery valve coupled to the delivery channel and designed to enable just a flow of fuel out of the cylinder, and a regulation device coupled to the intake valve to keep the intake valve open during a step of compression of the piston and hence enable a flow of fuel from the cylinder through the intake channel. The intake valve comprises a valve body that can move along the intake channel and a valve seat, which is designed to be engaged in a fluid-tight way by the valve body and is set at the end of the intake channel opposite to the end communicating with the cylinder. The regulation device comprises a control element, which is coupled to the valve body and is mobile between a passive position, in which it allows the valve body to engage in a fluid-tight way the valve seat, and an active position, in which it does not allow the valve body to engage the valve seat in a fluid-tight way. Coupled to the control element is an electromagnetic actuator, which is designed to displace the control element between the passive position and the active position.
In the case of (mechanical, electrical or electronic) malfunctioning of the variable-capacity high-pressure pump, the variable-capacity high-pressure pump itself could supply the common rail with an amount of fuel much higher than the necessary amount, thus causing a fast rise in the pressure of the fuel within the common rail. Once said situation of malfunctioning of the high-pressure pump has been detected, the low-pressure pump is immediately turned off in order to interrupt flow of fuel to the high-pressure pump and hence block the uncontrolled increase in the pressure of the fuel within the common rail. However, turning-off of the low-pressure pump has effect with a certain delay (equal to a certain number of pumping cycles of the high-pressure pump), and hence, without any further interventions of limitation, the pressure of the fuel within the common rail could reach values higher than the maximum value that can be physically withstood by the components of the injection system, with consequent failure of said components and outflow of fuel at a high pressure into the engine compartment. In order to limit the maximum pressure of the fuel within the common rail in the event of malfunctioning of the high-pressure pump, in known injection systems there is always present an electromechanical pressure regulator controlled by a control unit or else a mechanical pressure limiter.
However, coupling of an electromechanical pressure regulator or a mechanical pressure limiter to the common rail with the corresponding pipes for relief into the tank entails a non-negligible cost both in terms of purchase of the components and in terms of installation of said components; said cost is far from justified by the sporadic nature of the cases of intervention (i.e., cases of malfunctioning of the high-pressure pump that cause a sudden increase in the pressure of the fuel within the common rail).